Typically cellular telephones have a common antenna for receiving and transmitting signals within a relatively wide bandwidth. Various antenna arrangements are known in the art which have a wide enough bandwidth to cover both the transmitter and receiver frequencies used the FDD system.
U.S. Pat. No. 5,659,886 discloses in its preamble that in conventional mobile units for digital radio communication, both the receiver and transmitter are connected to a common receive/transmit antenna via a transmitting passband filter and a receiving passband filter. These filters may be fabricated as dielectric filters or acoustic wave filters. Since such components are difficult to fabricate as integrated circuits and also they are relatively bulky, this patent specification proposes that the transmitting bandpass filter be replaced by an isolator in order to reduce bulk. In the specific examples described, the common antenna comprises an external whip antenna. Isolators are themselves regarded as being inefficient devices because they can dissipate power reflected from the antenna.
Wireless terminals, such as mobile phone handsets, sometimes have an internal antenna, such as a Planar Inverted-F Antenna (PIFA) or similar. Such antennas are small (relative to a wavelength) and therefore, owing to the fundamental limits of small antennas, narrow band. However, cellular radio communication systems such as UMTS require a PIFA to have a fractional bandwidth of 13.3%. To achieve such a bandwidth from a PIFA for example requires a considerable volume, there being a direct relationship between the bandwidth of an antenna and its volume, but such a volume is not readily available with the current trends towards small handsets. Hence, because of the limits referred to above, it is not feasible to achieve efficient wide band radiation from small antennas in present-day wireless terminals.